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Trouble Shooter

If you want to predict the future, reviewing the past is an excellent place to start. Mode $06 provides a clear record of past results of noncontinuous monitor tests, possibly predicting future failures.

Trouble Brewing

We're working on a 1998 Toyota Camry with a 4-cylinder engine and over 150,000 miles on the odometer. The vehicle belongs to a regular customer who has been experiencing a recurring problem. Every so often and seemingly at random, the Check Engine light comes on and a DTC P0420 (catalyst efficiency below threshold) is stored in PCM memory. This has happened twice before, and each time I've examined the freeze frame data associated with the DTC. The DTC stores when the vehicle is going about 45 mph, fully warmed up and in closed loop. We've checked this vehicle's tailpipe emissions, and it's "blowing zeros." It would pass a tailpipe emissions test with ease. The last couple of times, I cleared the DTC and took the vehicle for a test drive. The Check Engine light didn't come back on and the customer was able to drive the vehicle for several months and several thousand miles before the light came back on. Based on our exhaust analyzer results, it's hard to believe that the converter isn't doing its job. I'm reluctant to replace any parts without some concrete evidence of an actual problem. Your suggestions?

Ray Thomas

Des Plaines, IL

In the days before OBD II, we might attempt to judge a catalytic converter's efficiency by measuring the temperature before and after the converter with a noncontact thermometer. If the temperature was hotter at the exit of the converter than it was at the entrance, we assumed that some actual catalyzing was going on inside the converter. If it wasn't too difficult, we might also use an exhaust analyzer to measure the emissions before and after the converter. This could be done by measuring the emissions before the converter got warm enough to "light off," or by tapping into the exhaust before the converter for a sample. The before sample was then compared to the after sample to determine catalyst efficiency.

As you know, OBD II doesn't directly measure tailpipe emissions to determine catalyst efficiency. Instead, it relies on two "spies" in the exhaust system: the front and rear oxygen sensors. The front sensor measures the oxygen content of the exhaust before the converter and the rear sensor checks things after the exhaust has been treated by the converter. Comparing the two allows the PCM to determine the converter's capacity to store oxygen, which it needs to catalyze the exhaust into mostly water and CO2 emissions.

It may seem counterintuitive, but the PCM doesn't really care what the vehicle's actual exhaust emissions are. As long as it remains convinced that the catalyst is performing effectively, it makes the assumption that emissions will remain low. And it must be right most of the time, because several states have adopted so-called plug-in OBD II tests for their vehicle emissions test programs. If the PCM on an OBD II-compliant vehicle is happy (no DTCs and most or all monitors run to completion), the folks at the test center are happy, too. Vehicles can be tested more quickly, and there's no need to put them through a dynamometer-based IM240 test.

The catalyst monitor on your customer's Camry is a noncontinuous monitor. It runs its tests only occasionally, and only when certain enabling criteria have been met. For example, if the vehicle has any outstanding DTCs or if one or more of the other monitors has not run to completion, the catalyst monitor probably won't run. If there are no stored DTCs and no MIL, there would be no outward indication that a problem existed. But if your state uses a plug-in OBD II test, your customer might run into problems getting the vehicle to pass if too many monitors are incomplete.

Start by checking the monitors. If the oxygen sensor and catalyst monitors are both incomplete, find out what's keeping the oxygen sensor monitor from running. If it's incomplete, the catalyst monitor may be waiting around until the oxygen sensor monitor completes before it runs its own test. The P0420 may result from the rare occasion when the oxygen sensor monitor actually completes and the catalyst monitor has a chance to do its thing.

Does the Camry still have its original front and rear oxygen sensors? If so, they probably have slowed considerably since new, and shouldn't be relied upon as expert witnesses; their testimony may be unreliable. The catalyst monitor may arbitrarily adjust the fuel mixture to see how the oxygen sensors respond during a test. Rapidly changing the mixture from lean to rich should provoke an abrupt change in the front sensor's response, while the rear sensor's response should be muted by the oxygen-storing influence of the converter. If the front and rear sensors respond similarly, the PCM will assume the converter isn't up to snuff. But if the front sensor has become lazy with age, its response to the mixture change may not be quick enough to convince the PCM.

If your scan tool supports it, Mode $06 is an excellent place to check the result of noncontinuous monitor tests. Most monitor tests assign minimum and maximum values. As long as the result is within that range, the PCM is happy. But if a result is at the edge of the acceptable range, it's an indication of impending problems. The catalyst may have passed the most recent monitor test, but if its performance is marginal and the results are close to the cutoff, it may fail the next time.

If both oxygen sensors are new or known-good, your customer is going to have to face the music and consider the possibility of converter replacement. The PCM can use only the data it collects from its sensors. And it won't be happy as long as the numbers occasionally fail to add up, regardless of what your exhaust analyzer tells you.